Waste determining leach rates

Method for Determining Leach Rates of Simulated Radioactive Waste Forms... 

For this reason, additional studies on carbon tetrachloride flux rates into and out of surface water, as well as refined quantitative estimates of aquatic fate processes would be valuable. The chemical is expected to evaporate rapidly from soil due to its high vapor pressure and may migrate into groundwater due to its low soil adsorption coefficient. No data are available on biodegradation in soil. Additional studies to determine degradation rates and the extent to which adsorption has occurred would be useful. These data are also useful in evaluating the impact of carbon tetrachloride leaching from hazardous waste sites. 

There are a number of methods for determining the chemical durability of waste forms. Different tests determine either the differential leach rate of individual constituents (in g-m 2-d ) or the cumulative leaching (in g-m 2, or mol% or wt%). The differential leach rate is given by... 

Owing to the use of these different methods, the results are often difficult to compare. Different procedures, however, are required to leant how waste-form materials behave under different conditions. Unless specified otherwise, all leach rates given in this chapter represent values determined with deionized water as the leachant. 

Most leach rate measurements of both matrix elements and radionuclides were performed at 90 °C using MCC-1 or PCT tests. According to these tests, leach rates range from 10 1 to 10g m 2 d (Lutze 1988). For example, the mass and elemental leach rates (in g-m 2-d ) for the PNL 76-68 glass containing 33 wt% waste oxides were determined at mass - 0.42, Ca - 0.068, Cs - 1.03, Mo - 1.40, Na - 1.32, Sr - 0.075, B - 1.12, and Si - 0.73. These values are typical for borosilicate waste glass as measured by the MCC-1 procedure (90 °C, 28 d). Leach rates of Fe-group elements and ACTs under the same test conditions are considerably lower (10-3 and 10 4g-m 2-d , respectively). 

The release rate was determined for 10 radioisotopes from fully radioactive waste glasses in deionized water for a period of 1.75 years. For cesium and strontium, good agreement exists between the leach rates for simulated and fully radioactive glass of the same composition. 

Gross dissolution of the waste can be measured from the loss of weight of the sample or by determination of the appearance in the leaching medium of the major matrix constituent e.g. glass). However, the selective leaching of important fission product elements has been observed in this work to be significantly different from that of the bulk waste matrix. The units normally used to describe leach rates, g/cm day, appear to imply... 

Leach rates for elements other than those listed in Table II can also be determined by this method. In fact, any element in the periodic table that is solid at room temperature and has an activation product with a half-life sufficiently long to allow leach testing can be studied with this technique. This method can also be applied to the study of the leach rates of alpha emitting actinides present in waste. In this case, standard carrier-free radiochemical procedures, coupled with low background alpha counting, would be invoked. 

Solidification/stabilization is a promising treatment technology for containing and immobilizing dredged material contaminants within a disposal site. While solidification/stabilization is not a solution to every disposal problem, the technology offers improved physical characteristics that reduce the accessibility of water to contaminated solids and reduced leachability for many contaminants. Leaching tests must be able to determine the (a) compounds that can be released from the waste (b) maximum concentration of these compounds in the leachate (c) quantities released per unit mass of waste (d) release rate of these compounds and (e) effects of a co-disposal of the waste. 

Of these three processes, some have received more study than others. There have been a number of determinations of leach rates (step 1) particularly from borosilicate-glass waste forms (see, for example, 1, 3), but there has been minimal effort to deter-... 

Alkalinity titrations of the 15 sequential batch stabilized/solidified waste extraets and of the acetic acid digestates were performed to determine the buffering capacity of the fixed waste and the rate of leaching of alkalinity from the waste. Fig. 1 shows the pH and alkalinity releases associated with one series of extractions. These analyses show a total buffering capacity of the stabilized waste ranging from 17.6 to 19.9 meg/g (dry weight) of fixed waste, with an average of 18.3 meq/g. 

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